Storm impact and evolution of a mangrove-fringed chenier plain, Shoal Bay,Darwin, Australia

Storms are considered to have significant impacts on the development of che nier plains, particularly through the devastation of mangrove vegetation, b ut also in terms of winnowing sand and shell from mudflats and forming chen ier ridges. Shoal Bay, in the Beagle Gulf, northern Australia, contains a s mall chenier plain, which was struck by a severe tropical cyclone, Cyclone Tracy, on Christmas Day 1974 as it devastated the city of Darwin. The morph ology, vegetation and stratigraphy of the plain are described. The plain is underlain by lower intertidal sand with shell hash. A radiocarbon age of 6 130 years BP, indicates mangrove colonisation of this flat at the time that sea level stabilised around its present level after the postglacial transg ression. Subsequently, progradation of mudflats has occurred, especially ar ound 2300 years ago, and shells of this age are found both in growth positi on from within the mudflats, and incorporated into the shelly chenier ridge s. A further phase of build-out, and subsequent erosion is reflected by stu mps and in situ bivalves being excavated presently on the foreshore, radioc arbon dating about 900-1100 years BP. Within this context of evolution, Cyc lone Tracy can be seen to have had severe consequences upon the patterning of mangroves, with extensive windthrow of Ceriops, and defoliation of Rhizo phora at the western end of the plain. Recovery of the mangrove vegetation has been gradual, and is still incomplete. However, the storm had little in fluence on the pattern of landform development. Indeed, the aerial photogra phic sequence indicates a trend by which sand shoals are gradually reworked landwards, in places moving through mangrove vegetation. The study suggest s that rather than pronounced regional episodes of alternative mudflat buil dout and erosion, both processes occur simultaneously at different points a long this foreshore. (C) 1999 Elsevier Science B.V. All rights reserved.